Plenty of fast-food brands are jumping on the healthy bandwagon these days. Sandwich chains like Subway and Così are particularly keen on offering low-calorie, health-conscious menu items, and several others have followed suit. While it’s certainly a good sign that such influential fast-food companies are taking the initiative to help Americans maintain a more balanced diet, it’s time to take a closer look at just how healthy the options are at national sandwich chains.

What makes for the "ideal" healthy sandwich, nutritionally speaking? According to nutritionist Kelly Aronica, there are many factors to consider. Based on a diet of roughly 1,800 calories per day (for those people trying to lose weight, or closer to 2,000 for those trying to maintain their current weight) a reasonable lunch should hit close to 500 calories. In this case, the sandwich should have about 400 calories, leaving 100 for sides and a drink. When making a sandwich at home, it’s easy to stay within this range using about four ounces of turkey, one slice of cheese, two slices of standard sandwich bread, and mustard (but no mayo). "Beyond that, it gets tough," says Aronica, "Flour tortillas are generally higher in calories than two slices of bread (though of course it depends on the bread). And adding any mayo or dressing will likely put it over the top." And things get even trickier when it comes to eating healthy when grabbing lunch from a fast-food chain.

But taking calories into account is just the tip of the iceberg; there are plenty of other variables that contribute to a healthy sandwich — like the breakdown of calories from fat, which should max out at about 30 percent overall, or roughly 15 grams of fat. In terms of saturated fat, a reasonable lunch should include no more than 4.5 grams or so in a 400-calorie sandwich. The final factor to consider is the sodium content — the daily recommendation is 2,300 milligrams, so try to choose a sandwich that stays close to 680 milligrams. Sodium is particularly sneaky when it comes to sandwiches because many deli meats tend to be positively full of the stuff, so be careful.

Using these parameters as a guide, how did chains like Panera Bread, Arby’s, and Quizno’s fare? The results may surprise you. Panera has three menu items that fit the bill, which believe it or not, ties it with Arby’s in terms of options. Quizno’s has one ultra-healthy sandwich on the menu, the same as the likes of Potbelly and Togo’s.

While the sodium content tended to be the killer on most of these restaurants' menus, there were a surprising number of national chains that couldn’t cut the mustard at all. Jimmy John’s and Blimpie, for instance, didn't have any that qualified. That being said, take a look through the fast-food sandwich shops that offer the healthiest options in the country.